This invention relates to a semiconductor laser having a buried heterostructure and more particularly to a semiconductor laser capable of oscillating two independent laser beams admitting of modulation.
Already known is a light-emitting diode device wherein a single semiconductor element enables multiple light beams having different wavelengths to be simultaneously emitted, for example, "Multi color light-emitting diodes" set forth in the U.S. Pat. No. 3,611,069. With a semiconductor element disclosed in this literature, two closely arranged light beams having different colors are simultaneously issued from the PN junction. However, this disclosed device is net adapted to be used intact with a semiconductor laser, because current tends to be dispersed. Another known 2-wavelength type semiconductor laser is that which is disclosed in the literature "Appl. Phys. Lett 35 (8) 15 Oct., 1979" page 588 (FIG. 1). This proposed device, too, has the drawbacks that an oscillation threshold current tends to rise, presenting difficulties in the room temperature operation; and two laser beams oscillated from an active layer are spaced from each other at a wide interval, failing to be conducted into a single optical fiber bundle at the same time. Therefore, a single semiconductor laser alone cannot provide a transmitter used in the so-called wavelength-multiplexing optical transmission intended to increase an amount of transmission by simultaneously transmitting optical signals having two or more wavelengths. Consequently it is indispensable to apply a wave collector. Further where the above-mentioned transmitter is to be formed of semiconductor laser units, each of which oscillates only one laser beam, then it is necessary to provide a prism or interference filter for collecting laser beams emitted from these plural semiconductor laser units. However, any type of conventional transmitter has the drawback that it inevitably assumes an enlarged size due to the necessity of providing a wave collector. Moreover, such a transmitter provided with a wave collector has the defects that when the waves of laser beams are collected by the wave collector, then laser beams tend to be lost in the region of said wave collector; and a distance of transmission has to be shortened. Therefore, a conventional semiconductor laser has presented difficulties in constructing a compact wavelength-multiplexing optical transmitter which dispenses with a wave collector and minimizes the loss of laser beams.
Further, where the conventional semiconductor laser emitting a single laser beam is used as an optical transmitter, then the difficulties arise that if the semiconductor laser should fail, a fresh semiconductor laser has to be installed, and the repair of the defective semiconductor laser consumes a great deal of time.